1. Field of the Invention
The present invention relates to a semiconductor device, particularly to a semiconductor device using refractory metal as electrodes or the like, and to a process for manufacturing the same.
2. Prior Art of the Invention
In conventional semiconductor devices, use are made of, as materials of electrodes, wirings or the like, metal having a low melting point, such as aluminum (hereinafter referred to as "Al"), refractory metal such as molybdenum (hereinafter referred to as "Mo", tungsten (hereinafter referred to as "W"), tantalum hereinafter referred to as "Ta") or titanium (hereinafter referred to as "Ti"), or semiconductor materials such as polycrystalline silicon (hereinafter abbreviated as "poly Si").
For example, a semiconductor device using Mo is disclosed by Fumihiko Yanagawa et al., "(Invited) Mo-gate MOS metallization System", Proceedings of the 10th Conference on Solid State Devices, Tokyo, 1978; Japanese Journal of Applied Physics, Volume 18 (1979) Supplement 18-1, pp. 237-245.
Each of the above-mentioned materials has some advantages and disadvantages. Al has an advantage of having a low resistivity, but a disadvantage of having a low melting point of 660.degree. C., which imposes various restraints on the semiconductor device fabrication process including the step of annealing, which is usually required to be effected at about 1,000.degree. C.
Poly Si has advantages of the capability of resisting annealing at about 1,000.degree. C. and of having a good affinity to silicon used as a substrate, both of which give wide freedom in designing the semiconductor device fabrication process. Furthermore, poly Si advantageously forms a silicon dioxide (hereinafter referred to as "SiO.sub.2 ") film having a good electrical insulating quality on its surface easily by merely processing annealing in an oxidating atmosphere. Poly Si and SiO.sub.2 are both resistible to acid washing with an adequate mixed liquid of H.sub.2 SO.sub.4, HCl, HNO.sub.3, H.sub.2 O.sub.2 and so on (hereinafter simply referred to as "acid washing"). Therefore, an element surface can easily be cleaned. In view of the above, the use of poly Si as the material of electrodes, wirings and so on advantageously leads to a high yield in the semiconductor device fabrication. The resistivity of poly Si, however, is higher by the order of two or three figures than those of metals. This leads to an increase in propagation delay due to wiring resistance in a semiconductor device using poly Si as the material of electrodes, wirings and so on. Thus, difficulties have been encountered in realizing a high integration and high speed semiconductor device.
On the other hand, a refractory metal, e.g., Mo with a melting point of about 2,600.degree. C., is resistible to annealing at about 1,000.degree. C. Therefore, the use of a refractory metal like Mo as the material of electrodes, wirings and so on provides wide freedom in designing the semiconductor device fabrication process. Moreover, such refractory metals are low in resistivity, and, hence, facilitate a high speed operation of semiconductor device. In view of the above, semiconductor devices using a refractory metal as the material of electrodes, wirings and so on have been increasingly attracting attention. In spite of these merits, such semiconductor devices have not been able to occupy a leading position in the field of semiconductor technique, since the use of a refractory metal has not realized a semiconductor device having a structure with a stable insulating layer of good quality and a simple process of manufacturing the same, unlike the use of poly Si capable of forming thereon a stable thermally-oxidized SiO.sub.2 film with good quality.
A structure having an insulating layer such as an SiO.sub.2 film arranged on a refractory metal layer has been incorporated into some conventional semiconductor devices. However, such an SiO.sub.2 film is formed by chemical vapor deposition (hereinafter abbreviated as "CVD") method or the like (SiO.sub.2 film deposited by CVD method will hereinafter be referred to as "CVD SiO.sub.2 "), and, hence, the film is poor in quality. For example, the breakdown strength of the SiO.sub.2 film thus formed is lower than that of the thermally-oxidized SiO.sub.2 film. Moreover, in the case of CVD method, a CVD SiO.sub.2 film is deposited all over the whole surface of the substrate including refractory metal and is not selectively formed only on the surface of the refractory metal. Furthermore, it is difficult to deposit the CVD SiO.sub.2 with a uniform thickness over the surface of a step portion, where the CVD SiO.sub.2 is usually formed in an overhanging manner. This disadvantageously leads to frequent occurrences of short circuit or disconnection in a semiconductor device having a three-layer structure of a conductor layer/a refractory metal layer with a step portion/a CVD SiO.sub.2 layer as an insulating interlayer. Additionally, the deposition of CVD SiO.sub.2 on the surface of the refractory metal layer requires complicated and time-consuming procedures of lowering once the temperature inside a CVD apparatus, prior to placing a structure having the refractory metal layer in the CVD apparatus in order to avoid oxidation of the refractory metal, filling the apparatus with an inert atmosphere, raising the temperature inside the apparatus, and introducing a reactive gas into the apparatus. Also, the conventional deposition of CVD SiO.sub.2 involves the problems of breakdown strength of the resulting CVD SiO.sub.2 film and of existence of pinholes in the film. In order to solve the problems, a CVD SiO.sub.2 film must be thick, for example, with a thickness of 5,000 .ANG.. This results in difficulties in realizing a high density fabrication of semiconductor devices.
As an example of semiconductor devices using poly Si or a refractory metal as mentioned above as a gate electrode, a MIS type field-effect transistor (hereinafter abbreviated as "MISFET") with a structure as shown in FIG. 1 has been proposed.
In FIG. 1, a MISFET device comprises a substrate 111 having a source region 112 and a drain region 113 therein, and a gate electrode 115 of poly Si or a refractory metal provided on the substrate 111 with a gate oxide layer 114 therebetween. On the gate electrode 115 an insulating CVD SiO.sub.2 film 116 is formed. Contact holes 117 are provided through the CVD SiO.sub.2 film 116 and the gate oxide layer 114 on the both sides of the gate electrode 115. A source electrode 118 and a drain electrode 119 are so provided through the contact holes 117 as to be in contact with the source region 112 and the drain region 113, respectively.
The structure of FIG. 1 is formed by ion implantation process by using the gate electrode 115 as a mask to form the source region 112 and the drain region 113 positioned in self-alignment with the gate electrode 115, depositing the CVD SiO.sub.2 film 116, and forming the contact holes 117 by photolithographic and etching techniques. The CVD SiO.sub.2 film 116 is formed uniformly not only on the surface of the gate electrode 115 but also on the gate oxide layer 114, so that it is necessary to form the contact holes 117 in the CVD SiO.sub.2 film 116. A conventional photolithographic technique to be used in the formation of the contact holes 117 can attain only a limited precision and accordingly it is difficult to reduce the distance x between the side wall of the gate electrode 115 and the contact holes 117 to 1 .mu.m or less.
Furthermore, in such a structure as mentioned above, the distance x cannot be reduced in the case that a refractory metal is used as a material of the gate electrode 115, particularly because the CVD SiO.sub.2 has a low breakdown strength.
As described above, it is difficult, according to the conventional technique, to obtain a MISFET with a shortened distance x. Therefore, a high density fabrication of semiconductor devices cannot be realized. Furthermore, it is difficult in the conventional semiconductor device to realize a high speed operation of the semiconductor device because of a distance between the gate region under the gate electrode 115 and the contact holes 117.